In a combine harvester, after a crop has been cut, it is passed through a thresher which acts to separate the grain from the stalks or the straw. The straw is transported to the back of the harvester by means of straw walkers, while the grain and other crop particles are dropped onto one or more sieves. The material other than grain (hereafter further abbreviated with “MOG”) comprises chaff, short straw and other particles that the thresher has separated from the longer straw stems. The one or more sieves are reciprocated while air is blown upwards through it. The shaking of the sieve(s) distributes the grain evenly over the area of the sieve(s) and conveys the grain and MOG towards the back of the harvester. The grain that drops through the sieve(s) is collected in a sieve box using a grain auger (also called collecting trough) from which it is transported to a grain tank.
In GB 2224423, a combine harvester is described having a grain collecting and cleaning apparatus having a lower sieve element and three vertically spaced and offset upper sieve elements. The apparatus comprises a blower having a main outflow and an additional channel. The outflow openings of the additional channel are so arranged that a drop step between a stepped base disposed under a threshing basket and the front upper sieve element as well as the region below and behind the front upper sieve element are subjected to an air current. The middle and rear upper sieve elements are acted on by an air current from the main outflow channel of the blower. The grains separated out at the middle and rear upper sieve elements and other admixtures pass to the lower sieve element and are there subjected to a repeated sieving operation. The separated grains fall from the lower sieve element onto a grain base, which extends obliquely and at the lower end of which a grain worm is arranged operable to convey the grains to an elevator conveyor mounted laterally beside the machine frame. The stock mixture, which has been separated in the end region of the rear upper sieve element and the lower sieve element and consists predominantly of grain and short straw components, passes by way of a return base to a transverse conveyor worm, which conveys the stock mixture into a mixture elevator.
In this grain collecting and cleaning apparatus, the blower is thus located at the upstream end this apparatus. The disadvantage thereof is that air is directed to the rear, but not sufficient relative to the huge length. There is too much air in front and not enough at the rear. This means that the fan will blow grain and MOG with a strong air blast towards the rear and there MOG will fall through the cleaning sieve, as there is no wind. Consequently, no efficient use of the available surface for sieve activity is obtained.
In order to reduce that effect, one should increase the fan blast even more, ensuring the grain to be blown out of the combine. This however is a very unstable cleaning, hard to adjust, with a bad grain sample as a consequence. Trying to improve the grain sample will reduce the capacity and vice versa.
In U.S. Pat. No. 4,531,528, a cleaning shoe arrangement is disclosed having a generally horizontal and planar receiving element, receiving material from the threshing and separating sections and conveying at least a portion of it rearwardly to a chaffer having a downstream discharge edge and offset rearwardly and downwardly from the receiving element. A sieve immediately below the chaffer receives material passing downwards through the openings of the chaffer. A blower mounted ahead of and somewhat below the chaffer and sieve provides a rearwardly and upwardly directed air blast to the underside of those elements to assist their screening operation. Clean grain passing downwards through the sieve is intercepted by a forwardly and downwardly sloping floor which delivers it to a clean grain auger. At least some of the material carried rearwardly by the chaffer and sieve finds its way into the tailings auger. The receiving element further includes a foraminous portion having a series of conventional fingers extending at the downstream delivery edge thereof. A deflector carried by this receiving element extends laterally over its full width and extends downwardly and forwardly from adjacent this downstream edge. Closely spaced beneath and extending almost the full length of the sieve portion of the receiving element is a grain pan. The blower has suitable outlets so that air may be delivered rearwardly and upwardly to the underside of the main chaffer and the sieves, respectively, by a main duct and also rearwardly and upwardly onto the underside of the sieve portion of the receiving element by means of a forwardly directed duct cooperating with a deflector or scoop. A transversely extending sheet metal deflector portion is disposed so as to receive crop material passing over the rearward edge of the grain pan. The deflector surfaces cooperate to control the combined flow of grain from the grain pan and the downstream section of the sieve portion so that it passes well forward of the main sieve assembly and is released approximately vertically above a transversely extending downwardly and rearwardly sloping portion of the trough of the clean grain auger.
By locating a fan in the middle, a well balanced wind distribution is ensured, left to right, front to rear.
The problem with this cleaning shoe arrangement however is that clean grain from the receiving element that is falling onto the grain pan and that is guided by means of the deflector surfaces towards the clean grain auger, being the major part of the cleaned grain, falls in front of the main duct of the blower. This reduces the efficiency of the airflow and increases grain loss.